I've had my Nike+ for a fortnight now. When it does what it promises to do, it's a real joy. It adds a lot to my running to know how far I've run, what my pace is, and how all that compares to my previous runs. It's a great motivational tool too. (FWIW, I'm using it to train for my second half marathon.)
What I'm not really liking are the bugs with the hardware. How it should work is you plug it into a USB port, it automatically loads the software, and then automatically uploads your latest data to the website, roll credits and feel-good music. But so far I've had it refuse to recognise the device, insist I've plugged two devices in, recognise it but hang on uploading the data (requiring a computer restart), and worst of all, send the screen totally haywire. (I fixed that by looking up the soft reset procedure, but it hardly inspired confidence after my third use.)
Will they patch the firmware and make the bugs go away, or will it do its job for a few months and then completely die on me? I have no idea at this point, but for now I'm sticking with the pain because I like the gain.